1. Field of the Invention
This invention relates to a torsional vibration damper which is designed for installation in the drive train of a motor vehicle, in particular for installation in a clutch disc of a motor vehicle friction clutch.
2. Background Information
When a motor vehicle is travelling either at extremely slow speeds or at slow speed on a level road or on a slight uphill or downhill grade, the engine of the motor vehicle is subjected to a torque between approximately 20 Nm when accelerating and approximately 10 Nm when decelerating. Under these operating conditions, transmission or rattling noises can occur, in particular when the transmission is in first or second gear.
To suppress transmission noises of this type during operation at idle, an idle or pre-damper is used in addition to the load damper which is sized for the operation under load of a torsional vibration damper, which pre-damper is integrated into the clutch disc of a motor vehicle friction clutch, the spring stage of which was sized for torques which were significantly lower than the torque to be transmitted in operation under load. In known torsional vibration dampers of clutch discs, the angle of rotation was limited, or similar measures were used, to ensure that the working range of the pre-damper was limited to an angular range around the idle position of the torsional vibration damper. In the adjacent working angle range of the load damper, the pre-damper was bridged or bypassed.
In conventional torsional vibration dampers, the torque range which is experienced during travel at extremely slow speeds lies near the lower limit of the working range of the load damper. In this range, the torsional rigidity of the torsional vibration damper is already so great that the above-mentioned noises in the transmission can be excited.
Previous attempts to damp such transmission noises have included approaches which involve modifying the drive train by means of the installation of a flywheel which has two inertial masses, or alternatively, by active interventions in the engine control system, but such solutions are relatively complex and expensive.
It is known that multi-stage damper spring systems can be integrated into clutch discs of motor vehicle friction clutches. If it is only necessary to realize one pre-damper and one load damper, the space available, which is generally at a premium, is sufficient for the installation of the spring stages. The installation of more than two spring stages which must be sized for different working ranges is frequently impossible because of a lack of available space. Moreover, the tuning of an idle or pre-damper on one hand and a damper for operation at extremely slow speeds sized in the manner explained above for travel at extremely slow speeds is also problematic on account of the proximity both of the working angle ranges and the torque ranges, in particular if there is only a small amount of space available, in particular in the axial direction. Multi-stage dampers are also generally complex, time consuming and expensive to install. Known devices of this type also include torsional vibration dampers, the damping spring systems of which have a flattened spring characteristic.
DE-A-36 14 158, for example, discloses a torsional vibration damper which, in common apertures of a central disc on one hand and two lateral discs which are located axially one on either side of the central disc, contains pairs of damper springs which are supported on one another by means of a freely-moving intermediate piece. The intermediate piece, which can consist of intermediate discs on both sides of the central disc which are oriented so that they can rotate, has stops to limit the angle of rotation of the individual springs of each spring pair, to achieve a graduation of the spring characteristic.
DE-A-34 07 524 discloses that the working range of the pre-damper of a torsional vibration damper integrated into a clutch disc can be limited, and by applying a bias to the springs of a spring stage which is sized for load operation, it is possible to ensure that the initial torque of the load spring stage will be less than the final torque of the pre-damper spring stage. In this manner, the two spring stages which are connected in series can be made to work together in the transitional range, and a transition between the damper characteristic of the pre-damper spring stage on the one hand and the damper characteristic of the load damper stage on the other hand results which avoids sudden changes in torque.
To simplify the installation of multi-stage torsional vibration dampers, in particular for clutch discs, U.S. Pat. No. 4,603,767 teaches that the pre-damper can be realized in the form of an independently functioning module which can be handled separately, so that the pre-damper can be assembled and tested independently of the process of assembling and installing the load damper, before the pre-damper and the load damper are combined.